A display device such as a liquid crystal display needs high front side luminance. Therefore, a backlight device included in such a display device has an optical lens sheet laid thereon in order to improve the front side luminance.
A micro lens array sheet is an optical lens sheet used in a backlight device to improve the front side luminance. Such micro lens array sheets each for use in a backlight device are disclosed by JP 2004-145329 A, JP 2004-145328 A, JP 2004-126376 A, JP 2004-191611 A, and JP 2004-309557 A.
The micro lens array sheet is sheet-shaped and has a plurality of convex lenses (micro lenses) in a lattice arrangement on one surface. The micro lens array sheet for use in a backlight device improves the front side luminance by gathering light diffused from a light source by the plurality of micro lenses and emitting the light.
There is a method of producing an optical lens sheet called “roll-to-roll method,” according to which a molding pattern of ionizing radiation curing resin is formed on a sheet type substrate film using a molding roll having grooves or holes for forming lenses at the outer peripheral surface. According to the method, the lens forming grooves or holes formed at the outer peripheral surface of the molding roll are filled with the ionizing radiation curing resin. Then, the sheet type substrate film is wound around the outer peripheral surface of the molding roll filled with the ionizing radiation curing resin, and ionizing radiation is carried out. The ionizing radiation curing resin is cured by the ionizing radiation, so that a plurality of lenses (such as prisms and micro lenses) are formed on the substrate film. Through these steps, the optical lens sheet is produced.
When a micro lens array sheet is produced by the roll-to-roll method, a flat surface (hereinafter referred to as “flat part”) is present between micro lenses adjacent to each other on the micro lens array sheet. The flat part does not contribute to the collection of light at all, and therefore in consideration of the light collection effect, the flat part is preferably eliminated by placing the micro lenses in contact with each other.
However, when a micro lens array sheet is produced by the roll-to-roll method, it is substantially impossible to place the micro lenses in contact with each other because the edges of the lens forming holes on the molding roll must be in contact with each other in order to place the micro lenses in contact with each other. In this case, the edges of the holes on the molding roll have reduced strength and become more prone to cracks, so that the useful life of the molding roll is shortened. Therefore, when a micro lens array sheet is produced by the roll-to-roll method, a flat part having a prescribed width is provided between the micro lenses.
However, when a micro lens array sheet with such a flat part is used in a backlight device, luminance unevenness is generated. The flat part does not contribute to the collection of light and therefore directly emits light that comes perpendicularly from the plurality of line sources arranged in parallel to one another in the backlight device. Therefore, the luminance immediately above the line sources is greater than that of the other part, which results in luminance unevenness.
Note that JP 9-21903 A and JP 2000-249807 A are other patent documents having disclosure related to the present invention.